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A body is said to be at rest if its position does not change with time with respect to an observer (or a reference point). For example, the chairs of the dining table are at rest with respect to the dining table; the passengers sitting in a moving bus are said to be at rest with respect to the driver of the same bus, because their positions do not change with respect to the driver. This state is called state of inertia.


Inertia means a resistance to any change in motion. All objects have inertia. The larger the mass of an object, the more inertia it has. Newton’s First Law of Motion states that moving objects will continue moving in the same direction and at the same speed. It also states that objects at rest will remain at rest unless an outside force acts on them.

When the driver of a car puts on the brakes, the car slows down. The people in the car, however, will continue at the same speed the car was going because of their inertia. The seatbelt acts as a force to keep the people in place. Seatbelts help keep people from getting hurt when a car comes to a sudden stop.




It is the inability of the body to change its state of rest by itself.



  1.  A person standing in a bus falls backward when the bus suddenly starts moving. This is because, the person who is initially at rest continues to be at rest even after the bus has started moving.

 2.  A book lying on the table will remain at rest, until it is moved by some external agencies.

 3. When a carpet is beaten by a stick, the dust particles fall off vertically downwards once they are released and do not move along the carpet and fall off.

  (II) INERTIA OF MOTION  Inertia of motion is the inability of the body to change its state of motion by itself.   EXAMPLES 1. When a passenger gets down from a moving bus, he falls down in the direction of the motion of the bus.

 2. A passenger sitting in a moving car falls forward, when the car stops suddenly.

 3. An athlete running in a race will continue to run even after reaching the finishing point.


 It is the inability of the body to change its direction of motion by itself.



 1. When a soldier  fires a bullets on a fixed target.

 2. The pointing toward the poles by the magnet.

 3. The movement of space ships in space.


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